Semiconductor devices utilizing cadmium alloy regions



June 1, 19(55 G. l.. scHNABLE ASEMICONDUCTQR DEVICES UTILIZING lCADMIUMALLOY REGIONS Original Filed July 24, 1959 United States Patent O fait3,136,879 SEMICNBUCTGR DEVCQES UTILZENG CADE/HUM ALLY REGHNS George L.Schnable, Lansdaie, Pa., assigner, by mesne assignments, to PhilcoCorporation, Phiiadelphia, Pa., a corporation of Delaware riginalapplication July 24, 1959, Ser. No. 829,436. Divided and thisapplication Aug. 2S, 1961, Ser. No.

s claims. (ci. 14s- 1.5)

This application is a division of my copending application Serial No.829,436, tiled July 24, 1959, entitled Method of FabricatingSemiconductor Devices Comprising Cadmium-Containing Contacts, now PatentNo. 3,005,735.

This invention relates to improved semiconductor devices. Moreparticularly it relates to improved transistors and semi-conductordiodes of the type having rectifying alloy junctions, which can befabricated, stored or operated at higher temperatures than hasheretofore been posv sible with conventional transistors and diodes.

Heretofore the acceptor metal indium and alloys thereof have been almostuniversally used as alloying materials in forming the rectifyingjunctions of alloyand microalloy-junction transistors and diodescomprising n-type or intrinsic germanium bodies. However indium melts ata relatively low temperature (155 C.) and has relatively poor thermalconductivity (0.057 calorie centimeter/second centimeter2 C.). Becauseindium melts at only 155 C. transistors and diodes fabricated withindium cannot be used or ystored at temperatures substantially in excessof 100 C. For example, when transistors having indium alloy-junctioncollectors are operated at collector currents such that the averagetemperature of the transistor is in the vicinity of 100 C., thetemperature of discrete regions of the indium collector frequently risesto values in excess of 155 C. At these hot spots the indium melts andproceeds to dissolve the germanium contiguous thereto. As a result themolten indium frequently penetrates the very thin germanium base regionof the transistor and short-circuits the collector to the emitteropposing it, thereby destroying the transistor. Moreover, because indiumhas poor thermal conductivity, the indium-containing electrode removesheat slowly from the rectifying junction at which it is generated. Forthis reason and because indium melts at only 155 C. the maximum powerdissipation at which transistors and diodes fabricated with indium areoperated must be relatively low to avoid permanent damage to thesedevices by overheating.

Furthermore, because indium melts at such a low ternperature, it is notpracticable adequately to vacuum-bake an indium alloy-junction deviceduring its fabrication, thereby to drive out from its electrodes andsemiconductive body undesired contaminants such as occluded gases, saltsand solvents used during the fabrication thereof. As a result many suchunits have only relatively short operating lives, being graduallypoisoned by the contaminants still present -thereon after baking.

Accordingly it is an object of the invention to provide improvedsemiconductor devices.

Another object is to provide an improved process for fabricating suchdevices.

Another object is to provide alloy-junction semiconductor devices suchas transistors and diodes operable at ternperatures substantially higherthan those at which similar devices incorporating indium electrodes areoperable.

Another object is to provide alloy-junction semiconductor devices whoserectiiier electrodes can dissipate heat at a substantially higher ratethan can semiconductor` devices employing indium electrodes of the samesize.

3,1%,879 Patented June 1, 1965 Another object is to provide an improvedprocess for fabricating alloy-junction transistors and diodes operableand storable at high temperatures.

The foregoing objects are achieved by the provision of a semiconductordevice, e.g. a transistor or diode, comprising a body constitutedprincipally of germanium and having two contiguous regions within saidbody. In accordance with the invention one of these regions is oomposedof an alloy comprising cadmium and germanium and having a solidustemperature above C. Preferably this alloy consists essentially ofgermanium and cadmium; germanium, cadmium and tin; germanium, cadmium,tin and galli-um; germanium, cadmium, tin and aluminum; germanium,cadmium, tin, aluminum and galliurn; germanium, cadmium and gallium;germanium, cadmium and aluminum, or germanium, cadmium, aluminum andgallium. Such alloys have p-type properties and, where the other of saidcontiguous regions has n-type or intrinsic properties, provide arectifying junction in the semiconductor device. Devices according tothe invention which comprise other suitable germanium-cadmium alloys aredescribed hereinafter.

This novel device is fabricated in accordance with the invention byapplying a mass of cadmium or an alloy thereof having a solidustemperature above 155 C., to a surface region of the germanium body,heating this metal mass suiciently to form a liquid mixture between itand germanium of said body, and cooling the mixture below its solidustemperature thereby to form a recrystallized region in said body. In aform of this novel process especially well suited .to the production ofextremely thin recrystallized regions, substantially pure cadmium iscoated onto said surface region of said germanium body, eg. by jetelectroplating. Next a mass of an alloy which may be composed oftin-gallium, tin-cadmium, tin-aluminum, tin-aluminum-gallium, tin-cadmium-gallium, tincadmium-aluminum, or tin-cadmium-aluminum-galliumis applied to the cadmium coating. Typically this alloy mass is aglobule of solder coated onto a lead wire to be secured to the portionof the germanium body within which the junction is formed, and isabutted against the cadmium coating. The alloy mass is heated suicientlyto form a liquid mixture between this mass, the cadmium coating and athin portion of the germanium body therebeneath. This liquid mixture isthen cooled to a temperature below its solidus temperature therebytotorm in the germanium body an alloy-junction region. By cooling theliquid mixture almost immediately after it forms, a junction is producedjust beneath the surface of the semiconductive body, eg. within about0.001 mil thereof. As a result the shape of the junction conformsclosely to that of the body surface region beneath which it is formed.Moreover where the alloy mass employed contains a small amount ofgallium and/ or aluminum, the recrystallized region thus formed containsa relatively high ooncentration of the latter acceptor substance andhence has excellent hole-injecting properties despite its thinness.

Alternatively the junction can be formed by placing a pellet of cadmiumonto the surface of a germanium body, heating body and pellet to atemperature suliiciently high for the pellet to alloy with germanium ofthe body, and cooling the alloy to a temperature below its solidustemperature. Because the cadmium-germanium eutectic melts at 319 C.,i.e., 164 C. above the melting point of indium, connections can be madeto the cadmium electrode by solders having considerably higher solidustemperatures than those of the solders usable to secure conductors toindium-containing electrodes.

Because cadmium and the solder alloys listed above begin to melt attemperatures substantially above the melting point of indium,semiconductor devices fabricated 3 with them can be vacuum-baked atcorrespondingly higher temperatures; hence the surface of the germaniumbody can be rendered freer of contaminants than was heretoforepracticable in devices fabricated with indium. Moreover my novel devicescan withstand correspondingly higher operating and storage temperatureswithout undergoing dissolution of portions of their electrodes.Furthermore, because cadmium has a thermal conductivity of about 0.22calorie centimeter/second centimeterZ C., i.e. about four times higherthan that of indium, the resultant device can dissipate considerablymore heat per unit volume for a given permissible rise in thetemperature of its junction that can an indium device. As a result thepower dissipation ratings of transistors and diodes fabricated withcadmium alone or one of the above-listed cadmium alloys and of a givensize can be considerably higher than those of transistors and ydiodes ofthe same size fabricated with indium.

Other advantages and features of the invention will become apparent froma consideration of the following detailed description, taken inconnection with the accompanying drawings, in which:

FGURES 1 to 3 are cross-sectional diagrams of a transistor according7 tothe invention, at various stages in its fabrication;

FIGURE 4 is a cross-sectional diagram of another transistor according tothe invention, and

FIGURES 5 to 7 are cross-sectional diagrams of a diode according to theinvention, at various stages in its fabrication.

The partially completed transistor shown in FIGURE 1 comprises arectangular wafer of n-type germanium typically having a resistivity ofabout, l ohm-centimeter, a length of 70 mils, a width'of 50 mils and athickness of 5 mils. Wafer 10 has formed therein a thin base region 12,eg. by electrolytically jet-etching it in a manner such as to produceopposed coaxial depressions whose respective surfaces 14 and 15 havesubstantially plane regions parallel to and spaced from one another by avery small distance, e.g., about 0.1 mil. A base electrode 18, whichtypically is a nickel tab, is secured to one end of wafer 10 by a bodyof solder 20 producing a substantially ohmic contact, e.g. tin, leadcontaining about 0.5 percent by weight of arsenic, or lead containingabout 2 percent by weight of antimony.

In accordance with the invention, disks 22 and 24 composed ofsubstantially pure cadmium are applied to surfaces 14 and 16respectively of wafer 10, eg. by jet electrolytic plating. A suitableplating solution is described hereinafter. Disk 22, beneath which anemitter junction is to be formed, has somewhat smaller diameter thandisk 24, beneath Whichia collector junction is to be formed. Typicallydisk 22 has a diameter of 4 mils and disk 24 has a diameter of 6 mils.

Next an emitter microalloy junction is formed in the region of wafer 10beneath disk 22. To form this junction the end of a nickel lead wire 26having a globule 2S of tin-cadmium-gallium solder electroplated thereonis abutted coaxially against cadmium disk 22. Preferably the cadmiumcontent of globule 28 is between about 15 and about percent by weight,and gallium content is about 1.5 percent by Weight. Such a solder beginsto melt at about 170 C. A process for electrodepositing globule 28 onwire 26 is described hereinafter. Globule 28 is then melted by heatingit radiatively, or conductively by way of wire 26, e.g. to a temperatureof about 300 C., thereby to cause globule 2S, disk 22 and an extremelythin portion of the semi-conductive body lying beneath disk 22 to form aliquid mixture. Almost immediately after this liquid mixture is formedthe heating is discontinued and the mixture cooled below its solidustemperature. As a result of this cooling a recrystallized region 30which is only about 0.001 mil thick and contains a relatively highconcentration of gallium forms within a portion of wafer 10 underlyingdisk 22. Because of the relatively high concentration of gallium thereinregion 30 is characterized by excellent hole-injection propertiesdespite its extreme thinness. In addition lead wire 26 is stronglybonded to Wafer 10 at region 30 by a solder fillet 32.

Next the collector junction is formed by abutting coaxially againstcadmium disk 24 the end of a lead wire 34 having a globule 36 oftin-cadmium solder aflixed thereto, by heating globule 36 sufficientlyto form a liquid mixture between it, disk 24 and a thin portion of wafer10 therebeneath, and by cooling this mixture below its solidustemperature. Preferably globule 36 contains about 35 percent by weightof cadmium and is electrodeposited onto Wire 34 by a method describedhereinafter. By performing. these steps a p-type'recrystallized region33 (see FIGURE 3) is formed Within wafer 10 and lead wire 34 is bondedthereto by a solder fillet 40. The acceptor substance in this p-typerecrystallized region is the cadmium of disk 16 and globule 36.

By employing the foregoing process a transistor assembly is producedwhich can be subjected to further heat treatment, e.g. vacuum-baking, attemperatures substantially above the melting point of indium, and whichcan be operated at similarly higher temperatures and with higher powerdissipation than can a transistor of the same size which is fabricatedwith indium.

For jet-electroplating cadmium disks 22 and 24 onto wafer 10, anelectrolytic solution composed of the following substances and preparedin the following manner has been found to be particularly satisfactory:

An aqueous solution containing between 49 and 51 percent by weight ofcadmium fluoborate and having a density of about 1.6 grams permilliliter,

An aqueous solution containing about 15 percent by weight of sodiumdecylbenzenesulfonate,

Deionized water having a minimum specific resistance of 5megohm-centimeters at 18 C.,

Aqueous ammonium hydroxide, A.C.S. reagent, electronic grade, containing2S to 30 percent by weight of NH3 and a maximum of heavy metals (aslead) of l part per million, and

An aqueous solution containing 48 to 50 percent by weight of uoboricacid.

ln preparing this plating bath, 10 liters of the deionized water areadded to a thoroughly cleansed S-gallon Pyrex carboy. Next millilitersof the cadmium fluoborate solution are added to this water an-d themixture is diluted to 18 liters with more deionized Water. Theseconstituents are then mixed thoroughly by bubbling nitrogen gas throughthe solution for 10 minutes at a .rate suiiicient lfor good mixing.Preferably the gas should be cleansed by passage through a sinteredglass tilter before entering the solution.

After this mixing has been completed the pH of the solution is measuredand its value adjusted to between abo-ut 2.1 and about 2.4. Where the pHis below about 2.1 it is raised `by adding an appropriate amount of theammonium hydroxide solution. Whe-re it is above about 2.4 it is loweredby adding an appropriate amount of the fluoboric acid solution. Aftereach addition of one of these pH-adjusting reagents, the solution isthoroughly mixed by bubbling the nitrogen gas therethrough for at least10 minutes and the pH is then remeasured to determine whether thesolution now has the proper pH. After its pH has been adjusted, thesolution is filtered through paper into a clean S-gallon polyethylenec-arboy. Just before using the solution to jet electroplate the cadmiumdisks, 2.5 `milliliters of the sodium decylbenzenesulfonate solution areadded thereto. Nitrogen gas is then bubbled through the plating solutionfor 5 minutes to mix the sodium decylbenzenesulfonate solutiontherewith.

To electroplate disks 22 and 24 onto surfaces 14 and 16 respectively ofwafer 10, substantially coaxial jets of the plating solution aredirectedagainst these respective surfaces, and a potential differenceexceeding the deposition potential of cadmium is applied between thejets and wafer in a direction such as to .pole wafer 10 negative withrespective to the jets. As a result cadmium diks 22 and 24 areelect-rodeposited on surfaces 14 and 16 respectively.

Globule 28 of tin-cadmium-gallium alloy is affixed to an end of leadwire 26 by employing the process described and Aclaimed in United StatesPatent No. 2,818,375. For this purpose an electrolytic solution preparedwith the following constituents has been found satisfactory:

In preparing this solution, the 'above-listed salts, are added to theglycerine and the mixture stirred for about 35 minutes at roomtemperature. Thereafter, while undergoing vigorous stirring, thesolution Vfirst is heated to a temperature of between about 135 C. andabout `145 C. and is maintained thereat for ten minutes, and then isheated to a temperature of between about 158 C. and 162 C. and ismaintained thereat for about 5 minutes. The solution then is cooled toabout 120 C. and filtered under suction through a fritted glass iilter.To reduce the surface tension of the ltrate and to reduce the grain sizeof the metal deposited therefrom, a surfactant is preferably added tothe 'ltrate. This surfactant is -a solution composed of l5 grams ofydodecylbenzene sodium sulfonate, 35 milliliters of water and-suiticient glycerol to make 100 milliliters of solution. 3.3milliliters of this surfactant solution are added to the filtrate afterits temperature falls to about 80 C., and the mixture is stirred slowlyfor about 5 minutes.

To plate globule 28 of tin-cadmium-gallium solder onto lead wire 26, asuitable quantity of the above-described solution is established at atemperature of between about 130 and 140 C., while dry argon or nitrogengas is passed over the surface of the solution to prevent its acquiringmoisture from the room atmosphere. About 0.5 mil of wire 26 is thenimmersed in the plating solution and a potential diiference of about 13volts is applied between wire 26 and an inert anode also immersedtherein. Typically this anode is a rod compose-d of spectroscopicallypure graphic Under these lconditions globule 2S of a ternary alloycomposed of about to 20 percent by weight lof cadmium, about 1.6 percentby Weight of gallium and the remainder tin rapidly electrodeposits inmolten form onto wire 26. For example an ellipsoidal globule weighingbetween about 15 and 20 .micrograms and having a minor axis diameter ,ofabout seven mils deposits in about four seconds on Ia nickel wire havinga diameter of 1.5 mils.

The potential difference is then removed and the plated wire 26 takenout of the plating solution.

Wire 26 is now ready to be bonded to wafer 10 by practicing thesoldering and cooling rsteps already described above. Because theaforedescribed plating solution is relatively viscous at 140 C., a layerof it clings to globule 23 upon its removal therefrom. This layer servesIas an excellent flux for the succeeding soldering Istep and hence noadditional flux need be applied to globule 28 or disk Stannous chloride,anhydrous 120.0 Cadmium chloride, anhydrous, A.C.S. reagent grade 83.2Ammonium chloride, granular, A.C.S. reagent grade =163.0

The stannous chloride, cadmium chloride and Iammonium chloride are addedto the glycerine. The mixture is stirred 'for about 35 minutes at roomtemperature. Thereafter it is heated rto a temperatu-re of between aboutC. `and 145 C. and is maintained at this temperature for about 10minutes while being stirred slowly. The resultant solution is thenpermitted to cool to about 120 C. and is filtered under suction througha sintered glass lter.

To plate globule 36 of tin-cadmium solder onto the end of lead wire 34,a suitable quantity of the above-described solution is established at atemperature of about 160 C. About a mil of =wire 34 is immersed therein,land a potential difference of about 22 Avolts is applied between wire34 and an inert (eg. graphite) anode also immersed therein. Under theseconditions globule 36, composed of about 65 percent fby weight of tinand about 35 percent by weight or" cadmium and melting at about 177 C.,is electrodeposited in molten -form onto wire 34. The plating iscontinued for about two seconds. Then the potential difference isremoved and the plated wire -is taken out of the bath.

After globule 36 has been plated onto wire 34, lit is abutted againstcadmium disk 24, and the heating and cooling steps already described arethen performed. Again the layer of plating solution adherent to theglobule serves as a soldering ux.

The transistor assembly shown in FIGURE 3 is nOW cleansed as follows:

First the assembly is rinsed in a hot (100 C.) solution consistingessentially of glacial acetic acid (3 percent by volume) dissolved in1,2-propanediol. Next it is rinsed in deionized water maintained at 80C. and then is rinsed in deionized water maintained at about 20 C.Thereafter two jets of a one molar aqueous solution of potassiumhydroxide are respectively directed against fillets 32 .and 40 and theportions of wafer 10 adjoining them. To cause electrolytic etching ofthe portions of wafer 10 impinged by the jets a potential dif- 'ferenceis applied between 'lead lwires 26 and 34 and platinum cathodes immersedin the jets, in a polarity such as to pole Wire-s 2.6 and. 34 and Wafer10 positive with respect to each jet. referably the jets are directed soas to intercept only a small portion of each lead wire in order thatmost of the electrolyzing current flows through the interface betweenwafer 10 and the jets rather than directly into the lead wires. lyticetching the unit is again rinsed in deionized water. Thereafter it isVacuum-baked at a temperature of about 160 C. for about 2 hours. Thisbaking temperature is above the melting point of indium C.) and there-Ifore considerably higher than the temperature at which transistorsfabricated with indium can be baked without damage thereto. Because suchhigher baking temperatures can be used, the gases occluded in thesurfaces of germanium wafer 10 as well as undesirable solvent materialspresent thereon and in fillets 32 and 40' are more readily decomposedand driven oit, and a transistor having superior operatingcharacteristics and a longer life -is thereby obtained.

Although the foregoing description relates to the structure and methodof fabricating a novel microalloy transistor having a homogeneous,n-type semiconductive base Subsequent to this electro-` region 12, theinvention is not limited thereto. On'the contrary, devices according tothe invention may have a variety of specific structures and the processmay have a variety of specic embodiments. In this regard FIGURE 4 showsa portion of a microalloy diiused-base power transistor according to theinvention. The latter transistor comprises a wafer 50 composed of n-typegermanium having a region 52 of high resistivity, e.g. 20ohm-centimeters, and regions 54 and 56 of lower resistivity. Moreparticularly, each of regions 54 and 56, which typically are prepared bydiffusing an n-type dopant, eg. vaporous arsenic, into the surfaces ofthe wafer, has a resistivity of only about 0.0005 ohmcentimeter at theexternal surfaces of the Wafer. This resistivity increases in anapproximately exponential manner at increasing distances below thesurfaces of wafer 50 and tinally becomes equal to the high resistivityof region 52 at about 0.1 mil below 4the surface thereof. A method forpreparing such regions is described and claimed in the copendingapplication of Richard A Williams, Serial No. 669,852, led July 3, 1957,now abandoned and entitled Semiconductive Devices and Method for theManufacture Thereot.

As `shown in FIGURE 4, two opposing, substantially coaxial depressions58 and 60 are jet-electrolytically etched into wafer 50. Depression 53is shallow, its surface 62 lying entirely within region 54 of lowe-rresistivity, while depression 60 is deep, its surface 64 cuttingentirely through region 56 and well into region 52. In this regard,where a transistor capable `of operating at high collector voltages isdesired, the portion of surface 64 most closely approaching surface 62and within -which the collector junction is to be formed is `fabricatedto lie within high-resistivity region 52, as shown in FIGURE 4. Where atransistor operating at relatively low collector voltages is desired,the portion of surface 64 most closely approaching surface 62 isfabricated to lie within lower resistivity region 54. Preferably thoseportions of surfaces 62 and 64 within which lthe emitter and collectorjunctions respectively are to be formed are substantially plane andparallel to one another.

To provide a lbase connection to region 54 of wafer 50, a nickel disk 65is affixed to region 54, e.g. by jet-electroplating, and a lead wire 66is soldered to disk 65 with a cadmium-tin solder 68 containing about 35percent -by weight of cadmium.

A suitable solution for use in jet-electroplating nickel disk 65consists of:

Nickelous chloride (NiCl2.6H2O), A.C.S. reagent grade grams 20 Boricacid, minimum assay 99.5 percent by weight,

A.C.S. reagent grade do 2 The pI-I of the foregoing solution is adjustedIwith concentrated ammonium hydroxide or concentrated hydrochloric acidto lie between 6.6 and 6.9.

In accordance with the invention, emitter and collector junctions areformed in wafer 50 -inV the manner already described above with regardto the transistor assembly ofA FIGURES l to 3. In brief, cadmium disks(not shown) are jet-electroplated onto surfaces 62 and 64. The end of awire 70 coated with a solder containing 15 to 20 percent .by weight ofcadmium, about 1.5 percent by weight .of gallium and the remainder oftin is abutted against the cadmium disk plated on surface 62. Thissolder is then heated sutlciently to cause it, the cadmium disk and anextremely thin portion of region 54 lying therebeneath to form a liquidmixture, and this mixture is almost immediately thereafter cooled belowits solidus temperature,V thereby forming recrystallized region 72 landbonding lead wire '70 thereto 4by a solder fillet 74.

To form a collector junction and aiix a relatively mas- -sive silverstud 176 to the recrystallized region associated with this junction theend 78 of silver s-tud 76 is coated with a solder consisting essentiallyof about l60 percent by weight of cadmium and about 40 percent by weightof tin. End 78 is then abutted against the cadmiumdisk plated ontosunface 64 .and the solder is heated to a temperature sufficient tocause it, the disk and a thin portion of region 52 therebeneath to formaliquid mixture. Upon cooling this mixture, a recrystallized region l80containing cadmium as an lacceptor impurity is formed `and stud 76 :isybonded thereto by solder llet S2. Then the assembly is cleansed byelectrolytically etching .the portion of surface 64 adjoining solderrllet 82 with a jet of aqueous potassium hydroxide solution.Thereafterthe metal stud :is secured in int-imate thermal contact with`the metal casing (not shown) of the transistor.

During operation of the transistor, this casing serves to dissipate theheat developed at the collector junction and elciently transmittedthereto by silver stud 76. Such heat-dissipative mountings for-transistors are described and claimed in copending application SerialNo. 733,613 ofW. L. Doelp, Jr., and F. K. Clarke, ytiled May 7, 1958,now US. Patent No. 2,977,515 and entitled Semiconductor Fabrication.Because the heat-conductive silver stud 76 is secured to wafer 52 by atin-cadmium solder, heat is transferred to stud 76 much more eicientlythan is possible in a device in which lthe silver stud is secured by anindium solder. Accordingly the power transistor of FIGURE 4 can beoperated at higher` power levels than can a transistor of the samedimensions which is however fabricated with an indium solder.

The method of the invention is not limited tothe formation of rectifyingjunctions by alloying one of the aforedescribed solders with cadmiumdeposited on a germanium wafer, and a portion of the wafer underlyingthis cadmium. Alternatively a cadmium-germanium rectify- -ing junctioncan be formed by plating cadmium onto the germanium -body asaforedescribed and 4by heating 4the coa-ted cadmium and the body abovethe melting point of cadmium (321 C.) thereby to alloy the disk with thebody. As another alternative the junction can .be formed by placing apellet of cadmium or an alloy thereof, e.g. cadmium .containing a smallamount of gallium, aluminum, or gallium and aluminum, on thesemiconductive body and by heating body and pellet above -t-he meltingpoint of cadmium thereby to alloy the pellet with the body. The latterprocess `is now described in greater detail with regard to FIGURES 5, 6and 7.

In particular FIGURE 5 shows a vwafer 100 of n-type germanium on onesurface of which rests a pellet 102 of cadmium. Typically wafer 100 hasa resistivity of about one ohm-centimeter. Because cadmium hasrelatively weak acceptor properties Vwhich are readily masked by thepresence .therein of donor dopants, the cadmium of pellet 102 should beof high purity, i.e. the total concentration of donor substancesltherein should .be substantially less than that which would cause therecrystallized region formed by alloying pellet 102 into wafer 100 to ben-type. Where cadmium pellet 102 is composed of such pure cadmium arectifying junction is readily formed within wafer 100 by alloyingpel-let 102 with the portion of wafer 100 Y lying therebeneath. Thisresult is achieved by heating the wafer and pellet to a temperature (eg.350 C.) in excess of the melting point of the pellet (321 C.) and belowthe melting point of germanium (958.5" C.). Such heating is typical-lyperformed on 4a heating strip positioned beneath wafer or in an alloyingfurnace of conventional form. Under these conditions pellet 162 and aportion of germanium wafer 100 lying therebeneat-h form a liquidmixture. The heating is continued for a .time sucient to permit enoughgermanium to dissolve into the molten cadmium adjacent wafer 100 sothat, upon cooling the molten cadmium-germanium mixture below itssolidus temperature, a recrystallized p-type region 104 forms'in wafer100 consisting of germanium doped with cadmium and having a p-n junction106 (see FIGURE 6). To

complete the diode, a base tab 15.08 composed of nickel is secured tothe opposite surface of wafer 100 by a solder 110 which typicallyconsists of tin or one of the aforementioned lead-arsenic orlead-antimony alloys. In addition a lead wire .1.12 is bonded .tocadmium dot 114 by a solder fillet .116 composed for example of tin ortin and cadmium. Because the diode is fabricated with etais havingmelting points considerably higher than that of indium i-t is feasibleto operate the diode at temperatures considerably above those at whichindium-containing devices can be operated safely. Moreover because ofthe relatively high thermal conductivity of cadmium, these diodes candissipate the heat generated therein more efficiently than can a diodeof the same size utilizing indium as its alloying metal.

In the foregoing examples the solder used to form the microalloy emitterjunction of each of the transistors described has -been stated to becomposed of between about l and 20 percent by weight of cadmium, about1.5 percent by `weight of gallium and the remainder tin, an alloy havinga solidus temperature of about 170 C. However it is to be .understoodthat the constituents, of this solder need not necessarily be present inthe foregoing proportions. For example all of the cadmium may be omittedfrom the solder. Under these conditions a solder is obtained containingabout 98.5 percent by weight of tin and about 1.5 percent by Weight ofgallium ,and melting at about 230 C. Because this tin-gallium soldermelts at a relatively high tempera-ture, soldering therewith isfacilitated .by applying to the cadmium disk Iand the solder lglobule aflux consisting essentially of one part by weight of zinc chloride toone part by weight of water. By then heating the solder globule -to atemperature above 230 C., a liquid mixture between the globule, thecadmium disk and .a portion of the germanium wafer therebeneath isformed. By cooling this mixture below its solidus .temperature almostimmediately after i-t forms, an extremely thin recrystallized regionrich in tgallium is produced lin the wafer which provides a rectifyingjunction having excellent hole-injection properties.

Moreover where desired, the acceptor dopant aluminum may be substitutedfor the gallium in any of the aforedescribed tin-gallium andtin-cadmium-gallium solders. Because aluminum has a greater solidsolubility in germanium than gallium at typical soldering temperatures,e.g. about 300 C., the foregoing solders may contain less of thissubstance than of gallium. "Ihus to obtain junctions which inject holesetiiciently, the aluminum concentration in a tin-aluminum ortin-cadmium-aluminum solder need only be between about 0.05 and about0.2 percent by weight thereof, whereas the preferred range -of galliumconcentration in tin-gallium or tin-cadmiumgallium solders is betweenabout 0.5 and about 1.6 percent by weight. Furthermore gallium andaluminum may be used concurrently as dopants in any of theaforedescribed tin or tin-cadmium solders. To maintain the soldermechanically strong, the gallium content thereof should not exceed about1.6 percent by weight.

In addition any one of the foregoing solders can be used to form acollector junction as well as an emitter junction.

In the transistors of FIGURES l to 3 described above, the collectorjunctions are preferably formed by using a tin-cadmium solder containingabout percent by weight of cadmium and melting at about 177 C. Thissolder is employed because it contains somewhat less than the eutecticamount of cadmium and hence when melted tends rapidly to dissolve thecadmium disk therebeneath. Such rapid dissolution is desirable becauseit promotes rapid formation of the liquid mixture between the solder,disk and germanium portion therebeneath. However cadmium-tin soldershaving proportions differing from the foregoing can also be used. ThusWhere enhanced heat conductivity of the rectifier electrode is desired,this may be achieved by increasing the amount of cadmium in the solderabove 35 percent, e.g. 60 percent of cadmium as I0 used in fillet 82(see FIGURE 4). Furthermore the collector junction can be formed byalloying pure cadmium Iinto the semiconductor body. Alternatively acadmium surface-barrier contact can be substituted for thealloy-junction collector Contact of the invention.

Where electrodes melting at temperatures even higher than the eutectictemperature of tin-cadmium alloys are desired, other metals and alloysmay be employed as solders. Examples of these other solders are thecadmium-lead alloy, whose eutectic contains 82.6 percent by weight oflead and melts at 248 C., the cadmiumzinc alloy, Whose eutectic contains17.4 percent by weight of cadmium and melts at 266 C., thecadmium-thallium alloy, whose eutectic contains 82.9 percent by weightof thallium and melts at 203.5 C., pure thallium melting at 303.6 C.,and the cadmium-gold alloy, Whose lowest-melting eutectic contains 87percent cadmium and melts at 309 C. In addition each of the foregoingsolders may contain the dopants gallium and/or aluminum.

In each of the foregoing examples a solder globule has beenelectroplated onto a Wire lead prior to soldering. While this method hasproved in practice to be well adapted for use on an automated assemblyline and therefore is preferred, it is to be understood that theglobules may instead be separately prepared by non-electrolyticprocesses, eg. by melting together appropriate amounts of the soldermetals in a Crucible and thereafter by forming alloy pellets ofappropriate size from this melt. Then each pellet may be afxed to a leadwire by warming the pellet to its softening temperature andl bythrusting an end of the wire into it. Alternatively an uncoated leadwire may be abutted against the cadmium disk and the appropriate solderintroduced between disk and wire in any other manner desired.

Moreover in each of the foregoing examples the germanium body has beendesignated as n-type. However, alloy-junction rectifying electrodes ofthe abovedescribed type can also be formed readily in intrinsicgermanium. Moreover electrodes having substantially ohmic properties canbe formed on a p-type germanium body by practicing any of theaforedescribed processes according to the invention. In addition ann-type recrystallized region providing a rectifying junction can beformed in a body composed of p-type germanium by employing as thealloying material a cadmium alloy having a solidus temperature above C.and containing a sulhcient amount of a donor substance to more thancompensate for the acceptor properties ofrcadmium and other acceptormetals which may be present in the alloy. For example this alloyingmaterial may be an alloy of cadm-lum and antimony containing betweenabout 2 and about 5 percent by weight of antiniony and having a eutectictemperature of about 290 C., or the eutectic alloy of cadmium andarsensic, containing about 0.3 percent by weight of arsenic and meltingat about 320 C. The latter two alloys can also be used to form contactshaving substantially ohmic properties on n-type germanium. Alternativelythe arsenic or antimony can be added as a dopant to any of theaforedescribed tin-cadmium, lead-cadmium, thallium-cadmium, Zinc-cadmiumor gold-cadmium alloys.

While I have described by invention by means of specitic examples and inspecific embodiments, I do not wish to be limited thereto for obviousmodifications will occur to those skilled in the art without departingfrom the scope of my invention.

What I claim is:

y1. A semiconductor device comprising an n-type semiconductor body and ap-type alloy region integral with said body, said body consistingessentially of n-type monocrystalline germanium and said alloy regionconsisting essentially of germanium and a material selected from theclass consisting of (a) cadmium,

"e (b) cadmium and gallium, (c) cadmium and aluminum, (d) cadmium,gallium and aluminum, (e) cadmium and an element selected from the groupconsisting of tin, lead, zine, thallium and gold, and (f) cadmium,anelement selected from the group consisting of tin, lead, zinc,thallium and gold, and a metal selected from the group consisting ofgallium and aluminum. 2. A transistor comprising a body of n-typegermanium providing a base region, an alloy region on one side of saidbody and integral therewith providing a p-type emitter region, and asecond alloy region on the opposing side.

of said body and also integral therewith providing a ptype collectorregion, each of said alloy regions consisting essentially of germaniumand a material selected from the class consisting of (e) cadmium,

(b) cadmium and gallium,

(c) cadmium and aluminum,

(d) cadmium, gallium and aluminum,

(e) cadmium and an element selected from the group consisting of tin,lead, Zinc, thallium and gold, and

(f) cadmium, an element selected from the group consisting oi' tin,lead, zinc, thallium and gold, and a metal selected from the groupconsisting of gallium and aluminum.

3. A transistor according to claim 2, wherein said alloy regionproviding said collector region is a microalloy region consistingessentially of germanium, cadmium and tin.

4. A transistor according to claim 2, wherein said alloy regionproviding said emitter region is constituted of an alloy consistingessentially of germanium, cadmium, tin and gallium.

5. A transistor according to claim 2, wherein said alloy regionproviding said collector region is constituted of an alloy consistingessentially of germanium, cadmium and tin.

6. AV transistor according to claim 5, wherein said alloy regionproviding said emitter region is constituted of an alloy consistingessentially of germanium, cadmium, tin and gallium.

7. A transistor according to claim 2, wherein both of said alloy regionsare constituted of an alloy consisting essentially of germanium,cadmium, tin and gallium.

8. A transistor according to claim 2, wherein at least one of saidemitter and collector regions is composed of an alloy consistingessentially of germanium and cadmium.

References Cited by the Examiner l'. Phys. Chem. Solids, Vol. 8, pages59-65, January 1959.

Bull. Am. Physical Soc., vol. l, page 127, 1956.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner.

1. A SEMICONDUCTOR DEVICE COMPRISING AN N-TYPE SEMICONDUCTOR BODY AND AP-TYPE ALLOY REGION INTEGRAL WITH SAID BODY, SAID BODY CONSISTINGESSENTIALLY OF N-TYPE MONOCRYSTALLINE GERMANIUM AND SAID ALLOY REGIONCONSISTING ESSENTIALLY OF GERMANIUM AND A MATERIAL SELECTED FROM THECLASS CONSISTING OF (A) CADMINUM, (B) CADMIUM AND GALLIUM, (C) CADMINUMAND ALUMINUM, (D) CADIMUM, GALLIUM AND ALUMINUM, (E) CADMIUM AND ANELEMENT SELECTED FROM THE GROUP CONSISTING OF TIN, LEAD, ZINC, THALIUMAND GOLD, AND (F) CADMIUM AND AN ELEMENT SELECTED FROM THE GROUP SISTINGOF TIN, LEAD, ZINC, THALIUM AND GOLD, AND A METAL SELECTED FROM THEGROUP CONSISTING OF GALLIUM AND ALUMINUM.